是否可以将其他类型变量引入超类约束？ [英] Is it possible to introduce additional type variables into a superclass-constraint?

问题描述

在处理类型族时，通常使用等号约束来避免在签名中重复某些类型函数的名称：

  class Foo f where 
 type BulkyAssociatedType f :: * 
 foo :: BulkyAssociatedType f  - > f 
 ... 
 
 bar :: forall m f b。 
（Monad m，Foo f，b〜BulkyAssociatedType f 
，Monoid b，Monoid（m b）
）=> f  - > mf 
  

即使缩写未在签名本身中出现， 。

对于类，这显然是不可能的;

  class （Foo f，b〜BulkyAssociatedType f，Monoid b，...）=> Bar f 
  

抱怨类型变量 b 不是

是否有某种方法可以实现类似的目的，避免一些重复 - 样板文件？

解决方案

令我感到惊讶的是，我不知道你做了什么（我使用过相同的技术，并且知道它在 instance 声明中有效），但是似乎是一个长期的 GHC功能请求来支持这一点。

也许你可以使用

  ConstraintKinds  可以获得相同的好处： > { - ＃LANGUAGE TypeFamilies，FlexibleContexts，ConstraintKinds＃ - } 
 import Data.Monoid 
 
 class Foo f其中
类型BulkyAssociatedType f :: * 
 
类型B f =（Monoid（BulkyAssociatedType f））
 
 class（Foo f，B f）=> Bar f 
  

When dealing with type families, it is often handy to use equality constraints to avoid having to repeat some type-function's name in a signature:

class Foo f where
  type BulkyAssociatedType f :: *
  foo :: BulkyAssociatedType f -> f
  ...

bar :: forall m f b .
       ( Monad m, Foo f, b ~ BulkyAssociatedType f
       , Monoid b, Monoid (m b)
       ) => f -> m f

This works even if the abbreviation doesn't turn up in the signature itself, only in the constraints.

With classes, this is apparently not possible;

class ( Foo f, b ~ BulkyAssociatedType f, Monoid b, ...) => Bar f

complains about type variable b not being in scope.

Is the some way to achieve a similar thing, to avoid a bit of repetition-boilerplate?

解决方案

It surprised me to learn you can't do that (I've used the same technique and know it works in instance declarations), but there seems to be a long-standing GHC feature request to support this.

Maybe you can use ConstraintKinds to get the same benefit:

{-# LANGUAGE TypeFamilies , FlexibleContexts , ConstraintKinds #-}
import Data.Monoid

class Foo f where
  type BulkyAssociatedType f :: *

type B f = (Monoid (BulkyAssociatedType f))

class ( Foo f, B f) => Bar f

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